The present invention relates to grammars used in speech recognition. In particular, the invention relates to context-free grammars.
In speech recognition systems, a computer system attempts to identify a sequence of words from a speech signal. One way to improve the accuracy of the recognition is to limit the recognition to a set of selected phrases. This is typically done by limiting valid recognition hypothesis to phrases that are found in a context-free grammar (CFG).
In prior art speech recognition systems, context-free grammars were specified using the Bakus-Nauer Form (BNF) which identifies valid phrases using a two-part format. The left part of the Bakus-Nauer Form contains a non-terminal reference to the phrase, and the right part of the Bakus-Nauer Form contains a sequence of terminal and non-terminal nodes that represent the phrase. In this context, a terminal node is a generally a word and a non-terminal node is a reference to an additional phrase or word.
Although the Bakus-Nauer Form does allow indirect reference to a phrase, thereby eliminating the need to repeat phrases in the context-free grammar, it is still somewhat limited. In particular, the Bakus-Nauer Form does not provide an easy means for providing alternative words, optional words, transitional probabilities, semantic properties, or executable code in a phrase.
In the past, some attempts have been made to modify the Bakus-Nauer Form to allow for some of these items. In particular, some prior art systems have added diacritics to the Bakus-Nauer Form to indicate that some words are alternative, others are optional, or the weight that is to be attributed to the words. The use of diacritics, however, is less than ideal since their meaning is not readily apparent and their scope is difficult to determine. In addition, diacritics do not allow for the inclusion of code or semantic properties in the context-free grammar.
Because of these factors, context-free grammars are difficult to author and have limited abilities. As such, a speech recognition system is needed that can take advantage of a different context-free grammar format.